For those of you experimenting with what to use .. here you go.
I'm gonna try and make this clean but with your dirty minds that's going to be neigh on impossible! It's a bit like Goldielocks and the 3 bears.
I tried the coat hanger and felt I didn't have the necessary control to get it past the first valve. It was too thin and too stiff
Second attempt was with industrial thickness earthing cable. This was far too thick and didn't give me the necessary control
Finally, I stripped the ends off my trusty emergency cat5 computer network cable with the mini cable snips I always have in my boot

and hey presto. Just enough stiffness to make it through the valve and enough flexibility to bend slightly when needed. Pushed the cable the whole way through the drain and pulled it out at the end.
After all this, I didn't get the deluge of gunk I was expecting, but there was some none the less. I reckon a leaf or something was caught up originally. I didn't get to flush through water, this I will have to do tonight when I get home.
Thanks PJ for your help.
S